The present invention relates to correction of a measuring signal being derived from a two beam photometer. Photometers of the type improved by the invention are used for example for fluid analysis. Two beams are derived for example from a single source for radiation, and one beam penetrates a measuring sample (test fluid), while the other beam serves as a reference. The two beams are modulated accordingly and the difference in intensity is detected and measured electrically. A signal is provided accordingly which is proportional to the concentration of a particular constituent in the test fluid.
Two beam photometers as outlined above in general are used for example in gas analysis. The source of radiation provides here particularly radiation which is rich in infrared. In addition to the modulation as established by absorption, the beams are modulated through periodic interception. Also, one uses a pneumatic electric radiation detector which has a gas filled detection chamber for intercepting each beam, and the periodically interrupted beams as absorbed in these chambers produce corresponding pressure variations and pulses. These pressure variations are applied to a diaphragm of a capacitor, and the resulting capacitance variations are electrically detected; they are reflected in a measuring signal representing the pressure difference in the detection chambers, accordingly, that signal is proportional to the beam intensities prior to detection. For a device of this kind, see for example British patent No. 634,453.
It was found that such photometric gas analyzer does not have constant zero or null point and also varies in regards to its sensitivity. Even greatest care in the assembly and operation does not eliminate these variations. As a consequence, a semiautomatic test and calibration apparatus has been developed (see for example German patent 1,548,653) which tests such gas analyzer in predetermined intervals, so that sensitivity and/or zero point can be adjusted; particularly the adjustment is carried out in the amplifier circuit of the instrument. This test equipment is quite extensive and includes gas flow diverter, automatic control and gas adjustment devices.